1. Field of the Invention
The present invention relates to an optical system for use in light measurement of an eye fundus, especially of the human, for detecting pulse waves in the artery at the eye fundus or detecting oxygen saturation in the artery blood at the eye fundus.
2. Description of the Prior Art
In devices for the observation of the eye fundus, eye fundus cameras have been known. The eye fundus cameras are designed to take pictures of the eye fundus with a flash illumination which continues for a very short time so that they are not affected by the movement of the eye to be photographed. However, when it is desired to measure or detect optically pulse waves in the artery at the eye fundus or oxygen saturation in the artery blood at the eye fundus, the optical system for the eye fundus camera can not be used without experiencing noise problems caused by movement of the eye during a time period required for the measurement or detection.
The prior art optical system for an eye fundus camera is arranged such that the light for illuminating the eye fundus is introduced to the eye through a centerapertured reflector which is disposed in front of the eye with its reflecting surface being inclined with respect to the optical axis of the camera and the light entering the eye and reflected at the eye fundus is directed to a camera through the center aperture of the reflector. With this optical system, the ray, which has a ring shape in cross section when passing through the pupil of the eye because of the center apertured configuration of the reflector, is partially blocked or obscured by the iris defining the pupil, as shown in the lower view of FIG. 1 when the eye moves a little from the position as shown in the upper view of FIG. 1, even if the ring R of the cross section of entering light has a smaller diameter than the pupil. Accordingly, the amount of light entering the eye for the illumination of the eye fundus varies with the movement of eye. In case of eye fundus camera employing such an optical system, such variation in the amount a minor illumination light may be of problem, because picture taking is carried out within a very short time. However, in the case of measurement of a pulse wave or the oxygen saturation in the blood vessel at the eye fundus, a considerable time period is required for the measurement of the light reflected from the eye fund so that the variation of the eye-fundus-illuminating light due to the movement of eye during that time can not be ignored. Such variation of illuminating light will reduce the S/N ratio in the resulting light measurement.